The only instance which I have observed of erosion of the basalt contemporaneous with the operations of the river that spread out this conglomerate is to be found in the striking stack of Dùn Beag already alluded to.[251] This extraordinary monument of geological history forms an outlying obelisk which rises from the platform of the shore to a height of about 70 feet. Seen from the south-west, it appears to consist entirely of bedded basalt resting on some stratified tuff and shale which intervene between these lavas and that of the broad platform of basalt on which the obelisk stands. On that side it presents no essential difference from the structure of the Dùn Mòr to the west, save that the lower conglomerate of that outlier is here represented by fine sediment, and the upper conglomerate is wanting. The general aspect of this south-western front of the stack is shown in Fig. 272. If, however, we approach the rock from the coast-gully to the north, we form a very different impression of its structure. It then appears to consist chiefly of conglomerate with a capping of basalt on the top (Fig. 273). Not until a close scrutiny is made of the eastern and western faces of the column do the true structure and history of this singular piece of topography become apparent. [Footnote 251: This pinnacle of rock is referred to by Macculloch in his account of Canna, and is figured in Plate xix. Fig. 3 of his work already cited. But neither his description nor his drawing conveys any idea of the real structure of the rock.] [Illustration: Fig. 274.--Section of eastern front of Dùn Beag. _a_, Very shaggy amygdaloidal basalt; _b_, shales and tuff; _c_, slaggy and jointed basalts; _d_, conglomerate; _e_, prismatic basalt. The dotted lines indicate the supposed form of the ravine.]